PPC subarches

I'm trying to understand the PPC architecture.
What are all those subarchitectures (pmac, chrp, chrp-rs6k and prep)
and why are they special? Why do they warrant their own kernel?
Is the AmigaONE a subarch, or it's own flavor?
--
Cuba Clinton radar Ortega plutonium domestic disruption [Hello to all
my fans in domestic surveillance] Ft. Meade Uzi attack Saddam Hussein
Honduras Nazi subway Treasury
[See http://www.aclu.org/echelonwatch/index.html for more about this]